Orthogonal Frequency Division Multiplexing Communication Systems Research Articles

Robust Multiplexing Waveform Design for Integrated OFDM Radar and Communication via Complex Weight Optimization

Compared to traditional single orthogonal frequency division multiplexing (OFDM) radar and OFDM communication systems, integrated OFDM radar and communication systems have the advantages of improved sharing of scarce spectrum resources, a simple hardware structure and a reduced interference between signals. In this paper, a constraint relaxation-based robust OFDM multiplexing waveform (ROW-CR) design method for integrated radar and communication systems is proposed. Considering the influence of correlated clutter and jamming signals, transmission and reception models of different system platforms are established by allocating subcarrier complex weights of different antenna signals so as to determine radar conditional mutual information and communication channel capacity parameters. Meanwhile, a restricted closed model is introduced through the limit range of the target frequency response. Then, a robust OFDM waveform optimization problem for integrated radar and communication systems is constructed by the minimax criterion, and the closed-form solution is obtained by adopting an improved method based on trace function properties and constraint relaxation, resulting in a better radar and communication performance trade-off. In addition, a parameter hierarchical optimization-based robust OFDM waveform (ROW-PHO) design method is further explored to reduce the computational complexity, and this method can also ensure a low system performance loss. Finally, the numerical simulation results verify the effectiveness of the proposed methods.

Deep Learning Peephole LSTM Neural Network-Based Channel State Estimators for OFDM 5G and Beyond Networks

This study uses deep learning (DL) techniques for pilot-based channel estimation in orthogonal frequency division multiplexing (OFDM). Conventional channel estimators in pilot-symbol-aided OFDM systems suffer from performance degradation, especially in low signal-to-noise ratio (SNR) regions, due to noise amplification in the estimation process, intercarrier interference, a lack of primary channel data, and poor performance with few pilots, although they exhibit lower complexity and require implicit knowledge of the channel statistics. A new method for estimating channels using DL with peephole long short-term memory (peephole LSTM) is proposed. The proposed peephole LSTM-based channel state estimator is deployed online after offline training with generated datasets to track channel parameters, which enables robust recovery of transmitted data. A comparison is made between the proposed estimator and conventional LSTM and GRU-based channel state estimators using three different DL optimization techniques. Due to the outstanding learning and generalization properties of the DL-based peephole LSTM model, the suggested estimator significantly outperforms the conventional least square (LS) and minimum mean square error (MMSE) estimators, especially with a few pilots. The suggested estimator can be used without prior information on channel statistics. For this reason, it seems promising that the proposed estimator can be used to estimate the channel states of an OFDM communication system.

Construction and application of polar codes in OFDM underwater acoustic communication

A polar-coded orthogonal frequency-division multiplexing (OFDM) underwater acoustic communication system was developed, and a Monte Carlo (MC) method for constructing optimal polar codes was proposed to improve the performance of the system in this study. Several optimal polar codes with different code lengths and rates were constructed for a shallow-water acoustic channel. Simulations showed that the polar codes constructed using the MC method rendered superior performance than those constructed using the Bhattacharyya parameter and the Gaussian approximation method. In addition, shallow-water acoustic communication experiments were conducted at Xiamen port. The results show that the developed polar-coded OFDM communication system has excellent performance with 196 out of 200 frames achieving error-free transmission, which is much greater than the performance (157 out of 200) of the LDPC-coded OFDM communication system.

Intelligent Resource Allocation for IRS-Enhanced OFDM Communication Systems: A Hybrid Deep Reinforcement Learning Approach

Orthogonal frequency division multiplexing (OFDM) systems have been widely applied in practice since OFDM has diverse outstanding advantages. However, their performance improvement is confronted with bottlenecks. In this paper, in order to tackle this issue, intelligent resource allocation driven by reinforcement learning is studied in intelligent reflecting surface (IRS) enhanced OFDM systems. The system sum rate is maximized by jointly optimizing the subcarrier allocation, the transmit beamforming of the base station and the phase shift of the IRS. An intelligent resource allocation scheme based on combining deep Q networks (DQN) and deep deterministic policy-gradient (DDPG) is proposed to tackle the formulated challenging non-convex problems. In order to further improve the spectrum efficiency, spectrum sharing is considered in the IRS-enhanced OFDM system. The secondary users sum rate maximization framework is formulated by jointly optimizing the channel allocation, the transmit beamforming of the secondary base station (SBS) and the phase shift of the IRS. Dueling double deep Q networks (D3QN) and twin delayed deep deterministic policy gradient (TD3) are exploited to tackle the hybrid action space issue under interference. Simulation results demonstrate that our proposed schemes can significantly improve the transmission rate compared to the benchmark schemes.

An efficient reconfigurable peak cancellation model for peak to average power ratio reduction in orthogonal frequency division multiplexing communication system

<span lang="EN-US">The peak to average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) communication system will be reduced using reconfigurable peak cancellation (RPC). RPC will also aid in improves the error vector magnitude (EVM) and reduces adjacent channel leakage ratio (ACLR) in OFDM communication system. The proposed RPC design methodology and practical implementation using field programmable gate array (FPGA) are discussed. The proposed RPC has been demonstrated using VIRTEX-7 XC7Z100 dual-core FPGA device with less hardware difficulty and minimum utilization of FPGA resources. The proposed RPC improves the efficiency of OFDM communication process by reducing complementary cumulative distribution function (CCDF) with respect to instantaneous power in dB. A comparison analysis was done between the existing selective mapping (SLM) method with proposed RPS method with respect FPGA resource utilization. The proposed RPC is implemented using VIRTEX-7 XC7Z100 dual-core FPGA device. Its effectively utilizing sub-carriers, fast Fourier transform (FFT) filter, bandwidth, and sampling frequency. Due to parallel switching operation, it reduces the PAPR, ACLR and improves EVM in OFDM signal with less hardware complexity.</span>

Real-Time Adaptive Modulation Schemes for Underwater Acoustic OFDM Communication.

Adaptive modulation received significant attention for underwater acoustic (UA) communication systems with the aim of increasing the system efficiency. It is challenging to attain a high data rate in UA communication, as UA channels vary fast, along with the environmental factors. For a time-varying UA channel, a self-adaptive system is an attractive option, which can choose the best method according to the channel condition to guarantee the continuous connectivity and high performance constantly. A real-time orthogonal frequency-division multiplexing (OFDM)-based adaptive UA communication system is presented in this paper, employing the National Instruments (NI) LabVIEW software and NI CompactDAQ device. In this paper, the received SNR is considered as a performance metric to select the transmission parameters, which are sent back to the transmitter for data transmission. In this research, a UA OFDM communication system is developed, employing adaptive modulation schemes for a nonstationary UA environment which allows to select subcarriers, modulation size, and allocate power adaptively to enhance the reliability of communication, guarantee continuous connectivity, and boost data rate. The recent UA communication experiments carried out in the Canning River, Western Australia, verify the performance of the proposed adaptive UA OFDM system, and the experimental results confirm the superiority of the proposed adaptive scheme.

PAPR reduction of OFDM systems using H‐SLM method with a multiplierless IFFT/FFT technique

This study proposes a novel low-complexity algorithm for computing inverse fast Fourier transform (IFFT)/fast Fourier transform (FFT) operations in binary phase shift keying-modulated orthogonal frequency division multiplexing (OFDM) communication systems without requiring any twiddle factor multiplications. The peak-to-average power ratio (PAPR) reduction capacity of an efficient PAPR reduction technique, that is, H-SLM method, is evaluated using the proposed IFFT algorithm without any complex multiplications, and the impact of oversampling factor for the accurate calculation of PAPR is analyzed. The power spectral density of an OFDM signal generated using the proposed multiplierless IFFT algorithm is also examined. Moreover, the bit-error-rate performance of the H-SLM technique with the proposed IFFT/FFT algorithm is compared with the classical methods. Simulation results show that the proposed IFFT/FFT algorithm used in the H-SLM method requires no complex multiplications, thereby minimizing power consumption as well as the area of IFFT/FFT processors used in OFDM communication systems.

Experimental demonstration of underwater wireless optical OFDM communication system with a single SPAD receiver

Generally, only binary modulation formats, such as on–off keying (OOK) and pulse position modulation (PPM), can be detected by a single photon avalanche diode (SPAD). In this paper, we propose to use a single SPAD receiver to achieve underwater wireless optical communication (UWOC) with high-order modulation formats. The feasibility is theoretically analyzed, and firstly demonstrated in a proof-of-concept orthogonal frequency division multiplexing (OFDM) experiment. The proposed OFDM system using quadrature phase shift keying (QPSK) and 16 quadrature amplitude modulation (16-QAM) can work with less than 68 and 390 photons per symbol, and with receiver sensitivities below the forward error correction (FEC) limit as high as −84.5 dBm and −77.8 dBm, respectively. These extraordinarily sensitive results indicate that the proposed scheme is a promising solution to long-distance UWOC systems or internet of underwater things (IoUT).

EM-EKF fast time-varying channel estimation based on superimposed pilot for high mobility OFDM systems

The scarcity of wireless spectrum resources and high mobility have brought serious challenges to orthogonal frequency division multiplexing (OFDM) communication systems. To improve spectral efficiency and combat fast time-varying channel fading, this paper studies the OFDM system with high mobility based on superimposed pilots, and adopts a basis expansion model (BEM) to describe fast time-varying channels. An iterative channel estimation algorithm combining expectation–maximization (EM) and extended Kalman filtering (EKF) is proposed, which is called EM-EKF in this paper. It uses EM to calculate the parameter matrix, measured values and the covariance of the process noise of the unknown first-order time-varying autoregressive (TVAR) model to obtain the optimal estimation. To further enhance the system’s robustness for fast time-varying channel fading, EKF and smoothing algorithms are jointly used for time-varying channel tracking. The results show that compared with the existing representative channel estimation algorithms, the iterative EM-EKF algorithm proposed in this paper not only solves the problem of unknown prior information, but also significantly outperforms other non-iterative algorithms, and the performance of the proposed algorithm is similar to that of the iterative EKF algorithm, which is more suitable for the actual OFDM communication systems with high mobility.

Mobility aware of WDM-based CMO OFDM communication system

Abstract The continued increase in several mobile applications forces to replace existing limited spectrum indoor radio frequency wireless connections with high-speed ones. Visible light communications (VLC) technology has gained prominence in the development of high data rate transmission for fifth-generation networks. In optical wireless communications, light-emitting diode (LED) transmitters are used in applications that desire mobility as LED divergence enables larger coverage. Since each VLC access point covers a small area, handovers of mobile users are inevitable. Wavelength division multiplexing (WDM) can be used in VLC systems to tackle the above issue and to meet the increasing demand for indoor connectivity with high bit rates. In this paper, a new system architecture for WDM with coded modulated optical in orthogonal frequency division multiplexing (OFDM) VLC system in conjunction with red, green, blue, and yellow (RGBY) LEDs is proposed to reduce the impact of random receiver orientation of indoor mobile users over VLC downlink channels and improves the system’s bit-error-rate (BER) performance. Simulation results show that the proposed method is not affected by the user’s mobility and hence it performs better than other approaches, in terms of BER for all scenarios and at all positions. This study reveals that using WDM-OFDM-VLC with RGBY LEDs to construct a VLC system is very promising.

Iterative turbo receiver design for superposition coding based downlink underwater acoustic OFDM communication system

Superposition coding (SC) with successive interference cancellation (SIC) is considered a promising non-orthogonal multiple access (NOMA) technique for improving spectral efficiency recently. In contrast, the residual interference after SIC at the receiver severely degrades the BER performance when applied in underwater acoustic (UWA) channels. In this paper, we propose a receiver that combined SIC and turbo equalization for superposition coding based downlink UWA orthogonal frequency-division multiplexing (OFDM) system, where a near-far user pair is mainly considered. We design an SC signal detector based on the minimum mean square error (MMSE) algorithm and eliminate the far user information with high power after detection. After SIC, the turbo equalization composed of an MMSE equalizer and LDPC decoder is adopted for the near user symbols with residual interference from the SIC process. Simulation results and recorded data from the Yellow Sea, China, show that the proposed receiver has a performance gain of at least 1 dB under different modulation pairs compared with traditional SIC receiver based on maximum a posteriori probability principle (MAP SIC). The error-free two-user downlink communication result shows a maximum sum-rate of 8kbps over the horizontal range of downlink 3.55km has been achieved in the Yellow Sea test.

Flexible and fast multi‐level modulation format identification using optical orthogonal frequency division multiplexing redundancy subcarriers

AbstractCompared with the orthogonal frequency division multiplexing (OFDM) communication system, the subcarrier‐index modulation OFDM communication system can alleviate the problem of peak‐to‐average power ratio and the frequency offset sensitivity. Therefore, the problem of inter carrier interference (ICI) and low power of the radio frequency amplifier in OFDM communication system can be solved to some extent. A fast and flexible modulation format recognition (MFR) scheme utilizing OFDM redundancy subcarriers is proposed in order to solve the MFR problem in coherent optical OFDM transmission. The scheme uses the silent subcarrier position of OFDM to carry modulation format information (MFI) and the receiving end decodes the MFI index by comparing the modulus values of the redundant symbols. It can recognize the phase shift keying (PSK) and multiple quadrature amplitude modulation (M‐QAM) modulation formats block by block. The number of symbols required by the proposed MFR is equal to the number of OFDM subcarriers, and the average time to get the recognition result in a block is O(1). We numerically investigate the proposed MFR among 20 Gbaud 4/16/32/64 QAM and 8/16 PSK. The results show that the correct recognition rate of the proposed scheme is not affected by the modulation formats. In particular, the proposed MFR is a potential solution for future flexible optical networks.

Compressive Sensing-Based Radar Imaging and Subcarrier Allocation for Joint MIMO OFDM Radar and Communication System.

In this paper, a joint multiple-input multiple-output (MIMO OFDM) radar and communication (RadCom) system is proposed, in which orthogonal frequency division multiplexing (OFDM) waveforms carrying data to be transmitted to the information receiver are exploited to get high-resolution radar images at the RadCom platform. Specifically, to get two-dimensional (i.e., range and azimuth angle) radar images with high resolution, a compressive sensing-based imaging algorithm is proposed that is applicable to the signal received through multiple receive antennas. Because both the radar imaging performance (i.e., the mean square error of the radar image) and the communication performance (i.e., the achievable rate) are affected by the subcarrier allocation across multiple transmit antennas, by analyzing both radar imaging and communication performances, we also propose a subcarrier allocation strategy such that a high achievable rate is obtained without sacrificing the radar imaging performance.

An Adaptive Multi-Mode Underwater Acoustic Communication System Using OSDM and Direct Sequence Spread Spectrum Modulation

Dynamic underwater acoustic network is a research hotspot. Due to the complexity and time-varying of underwater acoustic channel, the performance of single-mode underwater acoustic communication system is often limited by the worst channel situation. Orthogonal signal division multiplexing (OSDM) is a new signal modulation technology, which can be regarded as a bridge between single carrier modulation and orthogonal frequency division multiplexing (OFDM) modulation. In order to improve the channel adaptive ability of underwater acoustic communication system and obtain a unified architecture of different carrier modulation, an adaptive multi-mode underwater acoustic communication system based on OSDM and direct sequence spread spectrum modulation (AMMUAC/OS) is proposed in this paper. Combined with spread spectrum modulation, the system can realize single carrier time domain spread spectrum (SC-TDSS), single carrier system with cyclic prefix (SC-CP), OSDM, multicarrier frequency domain spread spectrum (MC-FDSS), and OFDM communication system by adjusting the vector length M of OSDM. The addition of a variety of digital mapping methods enables the system to achieve multi-mode communication with a unified architecture. Based on the relationship between the OSDM vector length M and the underwater acoustic channel delay, a joint adaptive decision criterion based on the maximum channel delay and the channel signal-to-noise ratio (SNR) is proposed. Firstly, the OSDM vector length M is determined according to the maximum channel delay, and then the system communication mode is selected according to the channel SNR. Orthogonal matching pursuit (OMP) channel estimation algorithm is used to obtain channel information and minimum mean square error (MMSE) channel equalization algorithm is used for channel compensation. An adaptive multi-mode data frame structure is designed for the interaction between the sender and the receiver. The simulation results show that the proposed AMMUAC/OS can adapt to underwater acoustic channels with different channel delay and SNR.

Hardware-Efficient All-Digital Architectures for OFDM Backscatter Modulators

Orthogonal frequency-division multiplexing (OFDM) backscatter communication promises to enable ultralow-power wireless devices that are robust to time-varying multipath channels. Traditional OFDM transmitters require the use of power-hungry digital-to-analog converters (DACs) and vector modulators to realize the OFDM signal, adding to the complexity and power consumption of wireless sensor nodes. In this work, we compare three all-digital architectures for OFDM backscatter communication that use RF switches and discrete loads to implement digitally controlled single-sideband OFDM backscatter modulators. We present design analysis, including simulations and measurements, for a selected implementation using five subcarriers having binary phase shift keying (BPSK) modulation at a symbol rate of 250 kSymbols/s and a throughput of 1.25 Mbit/s with a modulator energy consumption of 160 pJ/bit. We also present a five-subcarrier over-the-air validation with 195-kb/s throughput. We demonstrate how the number of RF switch states and the choice of impedances impact the metrics of subcarrier interference ratio and sideband suppression ratio, and we explore how reduced-numeric-precision inverse fast Fourier transform (IFFT) structures impact the theoretical bit-error rate. The all-digital architecture and analysis presented in this article enable new avenues of low-cost, digital OFDM, and multiple-access backscatter communication systems for use in challenging multipath environments.

Joint low‐complexity equalisation and CFO compensation for DCT–OFDM communication systems based on SIC

In wireless radio communication systems, equalisation is one of the most important algorithms to improve the bit-error-rate performance. The linear zero-forcing equalisers suffer from noise enhancement. The linear minimum mean square error equalisers suffer from large complexity and require the estimation of the signal-to-noise ratio to work properly. In this study, the authors propose a joint low-complexity regularised zero-forcing equaliser with successive interference cancellation (JLRZF–SIC) based on discrete cosine transform (DCT) for orthogonal frequency division multiplexing (OFDM) systems. The proposed JLRZF–SIC scheme jointly performs the equalisation and carrier frequency offset (CFO) compensation with lower-complexity using the banded-matrix approximation concept. Simulation results show the important role of the proposed JLRZF–SIC scheme is to improve the system performance compared to the other equalisation schemes.

Focusing gain analysis of time-reversal precoding in MISO OFDM communication systems

Emerging communication systems can benefit from time-reversal (TR) technology thanks to its good spatio-temporal signal focusing effect. The recent advances in low-cost wideband devices fabrication further leverage the use of TR wideband communication systems. The TR is generally carried out in the time domain and the focusing effect of TR comes from the use of a high transmit rate back-off factor (BOF), which is the signal up-sampling rate. In spite of the widely-used orthogonal frequency-division multiplexing (OFDM) modulation, few works have investigated the frequency-domain (FD) TR precoding in combination with OFDM communication systems. Furthermore, most existing works on FD/TR precoding rely on multiple-antenna technology at the transmitter to create the focusing effect. In this paper, we investigate the focusing gain provided by TR precoding in multiple-input single-output (MISO) OFDM systems. In particular, we compare the communication performance of such system at the intended position and that at the unintended position. Based on the analysis, we demonstrate that increasing the BOF and/or the number of transmit antennas significantly improves the focusing effect at the intended position. In contrast, the unintended positions receive less useful power. We derive approximated mean-square-error (MSE) expressions of equalized received signals at both intended and unintended positions. The subsequent focusing gain is presented as a function of the BOF and the number of antennas, enabling us to gain insights in the contributions of each parameter to the system performance. Numerical simulations with multi-path Rayleigh fading channels are carried out to validate the MSE expressions.

Message authentication algorithm for OFDM communication systems

With the huge expansion in the telecommunications industry, the need for robust information security is becoming more critical than ever. Physical Layer Security has, recently, emerged as a promising candidate to secure emerging communication systems in a lightweight manner. This approach has proven to be superior to existing schemes, which has motivated many researchers to shift their work towards studying, understanding and exploiting the randomness of the physical layer. In this context, this paper presents a novel keyed-hash function for Orthogonal Frequency Division Multiplexing (OFDM) systems based on the physical characteristics and properties of wireless channels. More specifically, communicating entities sharing the same channel, extract common properties and mix them with a pre-shared secret key to produce a channel-based dynamic key. This key will later be used to generate the cipher primitives needed for performing source authentication and message integrity of the transmitted OFDM symbols. Since OFDM symbols, in the frequency-domain and time-domain, have different properties, the proposed keyed-hash function is modified and adapted to suit each case leading to two variants of the solution. To the best of our knowledge, this is the first work that proposes a keyed-hash function for complex OFDM symbols, based on physical layer conditions. Finally, different security and performance tests are conducted to prove the robustness and efficiency of this technique, in comparison to currently employed message authentication schemes.

Low-Complexity Doppler Compensation Algorithm for Underwater Acoustic OFDM Systems With Nonuniform Doppler Shifts

The Doppler effect severely degrades the orthogonality of orthogonal frequency division multiplexing (OFDM) systems, and is a severe problem in the underwater acoustic (UWA) communications, leading to nonuniform Doppler shifts for all subcarriers. In order to alleviate Doppler effect, resampling technique by oversized fast Fourier transform (FFT) can achieve fast and accurate resampling in frequency domain. Nevertheless, the accuracy of this conventional FFT based approach depends on the FFT size, which in turn increases receiver complexity. In order to overcome this problem, the Doppler compensation model in frequency domain is simplified by Taylor series expansion in this letter. Based on the simplified model, we propose the Doppler compensation algorithm by using FFT and interpolation to achieve higher compensation accuracy. Simulation results demonstrate that the proposed algorithm shows very similar bit error rate (BER) performance with that of Chirp Z-transform (CZT) based method. However, the complexity of our proposed algorithm is about half of CZT based method. Further, our proposed low-complexity Doppler compensation algorithm can also be applied to the sampling frequency offset (SFO) correction in terrestrial OFDM communication systems.

Design and implementation of reconfigurable DCT based adaptive PST techniques in OFDM communication system using interleaver encoder

Objectives: To propose Reconfigurable Discrete Cosine Transform (RDCT) in Orthogonal Frequency Division Multiplexing (OFDM) high speed communication system to improve efficient spectrum utilization and mitigation of jitter,selective fading effects. The adaptive PTS method reduces Inter-Carrier-Interference (ICI), Inter-Symbol-Interference (ISI), PAPR and the computational intricacy of the communication system. To analyse and compare the performance of proposed Reconfigurable method with existing conventional OFDM communication system. Methods: This proposed technique employs interleaver encoder with both PTS and adaptive PTS with the Reconfigurable Discrete Cosine Transform (RDCT) in OFDM Communication system. Finding: A case study has been carried out and the proposed method is found to be better in CCDF performance, reliability, flexibility and accuracy. To reduce the autocorrelation between every modulated data bit, Forward error Correction in comparison with the conventional OFDM communication system. Novelty: The recent high-speed communication system most commonly using Orthogonal Frequency division multiplexing (OFDM) to enhance the quality of service(QoS) and Bandwidth on demand. Even though the OFDM scheme offers many advantages the signal of OFDM consists of a noise that causes variation in the amplitude resulting in high peak to average power ratio (PAPR) and since OFDM is a multi-carrier system, it is more susceptible to signal drifts. Various methods to reduce the PAPR in OFDM signal are already proposed, in this paper, a technique called interleaver encoder-based Partial Transmit sequence (IEPTS) is reviewed and the new proposed technique employs interleaving encoder utilizing in both PTS and A-PTS with reconfigurable discrete cosine transform (RDCT) in OFDM high-speed communication system. Keywords: Interleaver encoder; adaptive partial transmit sequence (A-PTS);orthogonal frequency-division multiplexing (OFDM); reconfigurable discrete cosine transform (RDCT); partial transmit sequence (PTS)